I had one do that before,was like the clutches or something was sliping and it would bang bang bang till you let off..

drive it easy and it was fine,just could not get it in a bind and torque it

short while later it went out,the front band eat the drum up and it died

I allways wondered if it was the over running clutch slipping the rollers or the clutch packs slipping or broke drum or drums slipped foward and eating themselfs up

I know I seem to kill a lot of 904s and a few 727 with my driving

wont hurt to start checking around for another one,just in case a rebuild needs some hard parts inside